Document U.S. Pat. No. 6,317,690 describes a method of generating a flight plan using a digital model for the elevation of the terrain that is being overflown, known as digital terrain evaluation data (DTED®) by the person skilled in the art.
Such digital models are “closed”, since a pilot cannot modify the content thereof.
Various types of digital model are available as a function of their accuracy. Thus, a level 1 DTED® model uses a mesh with an accuracy of three seconds of arc, i.e. about 100 meters, whereas a level 2 DTED® model uses accuracy of one second of arc, i.e. about 30 meters.
Nevertheless, that document U.S. Pat. No. 6,317,690 does not describe flight path generation in detail.
Document FR 2 712 251 provides for calculating a hypothetical curve associated with a theoretical optimum path for steering an obstacle, and in the field ahead of the aircraft it makes provision for:                determining all obstacles situated in a search zone;        comparing the tops of said obstacles with the hypothetical curve;        defining as dangerous any obstacles having tops that are higher than said hypothetical curve; and        communicating information about the tops of said dangerous obstacles.        
According to that document, the hypothetical curve thus follows the terrain with a dynamic vector that is adapted to the aircraft.
Nevertheless, that method implies using telemetry sensors in order to pick up obstacles and their tops. That gives rise to effectiveness that fluctuates as a function of the sensors used.
The same applies to the device set out in document FR 1 374 954.
According to document WO 2004/095394, a pilot defines a plurality of waypoints in order to generate horizontal main segments.
Each main segment is then positioned vertically at a given height above the terrain, obtained with the help of an elevation DTED® digital model.
That first stage serves to obtain a hypothetical framework having a plurality of levels.
Thus, in a horizontal projection on the ground, the theoretical route to be followed is a broken line. Furthermore, in a vertical section, the theoretical route comprises mutually parallel horizontal segments, each horizontal segment presenting a level with a “staircase” profile.
During a second stage, computer means determine a flight path that is compatible with the capacities of the aircraft tending to follow said framework as closely as possible.
That method is well suited to conventional flights, but it is not suitable for tracking terrain at low altitude. In order to reach a result that is satisfactory, it would be necessary for the pilot to determine a very large number of waypoints, and that would appear to be incompatible with a rescue mission undertaken urgently.
Document FR 2 658 636 suggests determining an avoidance path by minimizing a performance index that is a function of the altitude of the aircraft, of a lateral offset between the position of the aircraft and a reference itinerary, and of a weighting coefficient.
Document US 2006/0031004 provides for establishing a path to minimize the exposure of an aircraft to a threat. Thus, a path is generated from a digital elevation model, and then a survival rate is determined based on exposure to a threat along that path.
Thereafter, an optimized path is generated from the path that gives the best survival rate.
Document US 2006/0031004 thus applies to tactical flights and not to rescue missions. The paths that are generated do not appear to be optimized for such rescue missions, in which rescued passengers ought not to be subjected to too many varied and sudden movements.
The same applies to Document U.S. Pat. No. 5,706,011.
Document EP 0 775 953 discloses a method during which a pilot determines waypoints, with computer means generating a path having varying slopes. Such a path is effective when visual flight rules apply, but does not appear to be suitable for instrument flying.
Finally, document U.S. Pat. No. 7,321,812 presents a method of transforming a terrain profile into a profile for tracking the terrain in flight as a function of performance parameters of a vehicle.